1. Technical Field of the Invention
The present invention relates to a method for using a laser to cut an indentation into a polyimide film. In particular is relates to a method of using a laser to form pockets in polyimide stencils used in the application of solder paste for surface mount electronic assemblies.
2. Description Of Related Art
Surface mount technology is used to mount electronic components on the surface of printed circuit boards or substrates by soldering the components to one or both sides of a substrate. The first step in mounting surface mount components to a surface board is to screen print with a stencil solder paste on the board where the surface mount components are to be positioned.
In the manufacture of stencils, surface mount land patterns referred to as footprints or pads arc cut from a stencil to define the sites at which components are to be soldered to a printed circuit board. It should be understood that the design of the land patterns is critical because it not only determines the solder joint strength but it also influences the areas of solder defects cleanability, testability and repair/rework. The accuracy with which the land patterns are cut out from the stencils used in the assembly of printed circuits has a direct bearing on the quality of the finalized product. It is important that the solder paste align with the location of the solder pad and it is necessary that the aperture or land patterns cut out from the stencil be accurate. The accuracy in combination with the minute size of the components used in surface mount techniques results in very small tolerances for error (in the order of 0.0005 inches). The size of the openings cut into the stencil may be in the order of 0.01 inches in size or less.
Chemical etching processes are commonly used to cut out the apertures to form the land patterns in the stencils. While etching processes are well known in the art, they typically involve placing a chemical resistive material over the metal stencil which has openings where the platforms or lands are to be located. Then an etching process etches out openings where the lands are located. Thereafter, the protective layer of plastic on the metal is removed from the metal stencil.
Newer procedures have been developed to cut out land patterns in metal stencils using YAG lasers. These procedures are highly accurate and relatively expensive when one considers that the cost of purchasing a YAG laser is currently in the order of $100,000 to $200,000. Further, the operating costs of YAG lasers are relatively expensive. The YAG lasers typically have a beam focal path of sufficient power to cut through stainless steel stencils having a thickness of 0.005 to 0.012 inches. Consequently, it is important that the edges cut through the metal stencil remain constant. YAG lasers have proven useful in this application.
Recently, a polyimide stencil has been introduced to the market that can be manufactured with a more cost effective low power CO.sub.2 laser as well the more expensive YAG laser. This polyimide stencil sold under the trade mark KEPOCH is described in detail in corresponding Canadian Patent application Serial No. 2,181,207 filed Jul. 15, 1996 by Keith C. Carroll and entitled "Polyimide Stencil for use in Electronic Assemblies and Method of Making Same". The polyimide stencil described in this patent application is for a single level stencil.
While the use of lasers is now known for cutting both polyimide film and stainless steel stencils, it should be understood that the lasers are employed to cleanly cut through the stencil and form the openings in the stencils. To facilitate the laser cut, it is known to direct a gas under high pressure at the point where the cut is to be made. The gas, commonly compressed air, is chosen to be at a sufficiently high pressure to blow away any dross formation along the edges of the stencil with the apertures are cut.
Metal stencils have been manufactured with multilevels of stencil thickness in addition to the through openings to accommodate selective printing which allows varying depths of solder paste to be deposited on the circuit board. Multilevel etching of a metal stencil is typically accomplished by chemical milling to first etch a large area, referred to in the industry as a "pocket", to a desired thickness for the components that require lower paste thickness. The pocket area is larger than the land pattern area of the component to prevent solder skipping and damage to the squeegee used in the printing process. These pockets are about 0.002 inches deep in the stencil and are etched through chemical processes from the metal stencil so that the thickness of the stencil for fine pitch components is less than for larger components. The pocket formed in the mesh about the fine pitch component is an additional 0.1 inches. The pocket is formed first and then the rest of the stencil apertures are formed in a normal fashion which could include either chemical etching or laser cutting. Multi-level etched stencils have the advantage of allowing varying thicknesses of solder paste to be applied in one application.
While etching of multi-level stainless steel stencils through chemical milling is known, chemical milling or etching of the polyimide material does not appear to be as easily attainable as chemical milling of stainless steel due to the manner in which the etching chemicals would attach the polyimide material. A discussion of chemical etching of polyimide film is discussed in "Accelerated Chemical Etching of Kapton.RTM. Polyimide Film" by J. A. Kreuz et all and presented at the IPC 25th Annual Meeting of April 1982. This paper briefly describes that high energy laser beams can be used to cut precise holes. It also teaches a demand for this cutting in polyimide films; however, there is no teaching on how to use a laser beam to mill polyimide film. Accordingly, there is a need for a cost effective, reliable method for milling pockets in polyimide stencils to provide the advantages of both polyimide stencil and multi-level stencils.